In a liquid chromatograph mass spectrometer (LC-MS) in which a mass spectrometer is used as the detector for a liquid chromatograph (LC), an ion source which employs an atmospheric pressure ionization method, such as electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) or atmospheric pressure photoionization (APPI), is used to ionize a compound in a liquid sample. In a mass spectrometer employing such an atmospheric pressure ion source, ions generated within an ionization chamber in which an ambience of substantially atmospheric pressure is present need to be introduced into a vacuum chamber in which a vacuum atmosphere is maintained. To improve the sensitivity of the analysis, it is particularly important to increase the amount of ions generated within the ionization chamber, and to improve the efficiency of introducing ions from the ionization chamber into the vacuum chamber.
A commonly known technique aimed at increasing the amount of ions generated within an ESI ion source, which is a typically used atmospheric pressure ion source, is to supply a stream of heated gas onto electrically charged droplets sprayed from an ionization probe to promote desolvation of those droplets. For example, in a device described in Patent Literature 1, a stream of heated gas is supplied so as to intersect a traveling direction of the electrically charged droplets sprayed from the ionization probe. In another device, described in Patent Literature 2, a stream of heated gas is ejected in a hollow cylindrical form coaxially with the spray flow of the electrically charged droplets ejected from the ionization probe, i.e., the flowing direction of the heated gas is the same as the traveling direction of the electrically charged droplets. Both of these configurations have been proven to be effective for increasing the amount of ions to be generated. At the moment, desolvation techniques using heated gas based on one of the two aforementioned systems are adopted in almost all commercially offered mass spectrometers equipped with atmospheric pressure ion sources.
In an atmospheric pressure ion source, the arrangement of the ionization probe and an ion introduction section (e.g., ion introduction tube or sampling cone) is normally determined so that the spraying direction of the droplets from the ionization probe extends orthogonally or obliquely to the direction of introducing ions into the vacuum chamber, in order to prevent large droplets among the sample droplets sprayed from the ionization probe from being introduced into the vacuum chamber. The ions generated from the sample droplets are drawn into the ion introduction section and carried into the vacuum chamber by a gas stream flowing from the ionization chamber into the ion introduction section mainly due to the differential pressure between the two ends of the ion introduction section.
The direction of the aforementioned heated gas ejected for promoting the desolvation is normally different from that of the gas stream flowing into the ion introduction section produced by the differential pressure. Therefore, the stream of heated gas has no effect of increasing the amount of gas stream flowing into the ion introduction section. In the case of the configuration described in Patent Literature 2, the stream of heated gas may be a gas stream which is orthogonal to the ion introduction direction in an area near the ion introduction port, i.e., a gas stream which flows in a direction which interferes with the introduction of the ions. Although the heated gas is effective for increasing the amount of ion generation, it cannot be considered to be effective from the viewpoint of improving the efficiency of introducing ions from the ionization chamber into the vacuum chamber.
One method for improving the ion introduction efficiency is proposed in Patent Literature 2, in which a voltage applied to the ion introduction port is adjusted to create an appropriate electric field in the vicinity of the ion introduction port so that the ions near the ion introduction port will be attracted and collected into the same port by the effect of the electric field. However, it is difficult to create an electric field which is strong enough to sufficiently collect the ions against the powerful stream of heated gas flowing in the orthogonal direction to the ion introduction direction. Accordingly, in such a method, it is difficult to significantly improve the efficiency of introducing the ions from the ionization chamber into the vacuum chamber.
In view of such a problem, the applicant proposes a novel configuration of an atmospheric pressure ion source as described in in Patent Literature 3. In a mass spectrometer described in Patent Literature 3, an auxiliary electrode having a cylindrical shape and a reflecting electrode also having a cylindrical shape are arranged anterior to a spray flow ejected from an ionization probe and concentrically about the central axis of the spray flow. The auxiliary electrode and the reflecting electrode are coaxially arranged with a predetermined space from each other, and an ion introduction port is arranged within a space between these two electrodes. The auxiliary electrode and an ion introduction section are grounded, while the reflecting electrode is supplied with a predetermined direct-current voltage having the same polarity as measurement target ions. As a result, a reflecting electric field which reflects ions and electrically charged droplets originating from sample components, being carried by the spray flow, is created within the space between the auxiliary electrode and the reflecting electrode, and a focusing electric field for focusing ions to the ion introduction port is also created in an area near the ion introduction port. Due to the effect of these electric fields, the ions originating from the sample components are separated from the gas flow in the spray flow and gathered around the ion introduction port, to be efficiently drawn into the ion introduction section.